1. Field of the Invention
The present invention generally relates to power combiners/splitters in a distributed or coupled line technology. Such devices are used to divide an input power between two balanced paths or to add two input powers in a common path. Such devices can generally be found in association with balanced power amplifiers, mixers, phase shifters, most often to combine several powers obtained from several different amplification paths.
2. Discussion of the Related Art
FIG. 1 shows, in the form of a block, a power combiner/splitter (COMB/DIV) 1. This circuit comprises an access IN, arbitrarily designated as an input, intended to receive a signal Pin, the power of which is to be distributed (or to provide a combined signal), and two accesses OUT1 and OUT2, arbitrarily designated as outputs, intended to provide distributed power signals Pout1 and Pout2 (or to receive signals, the powers of which are to be combined) in phase or in phase quadrature. Circuit 1 has the function not only of equitably distributing power Pin between output accesses Pout1 and Pout2 in phase or in phase quadrature, but also of ensuring the isolation between these accesses. Such a device most often is bi-directional, that is, it can be used, according to its assembly in an electronic circuit, to combine two powers Pout1 and Pout2 in a single signal Pin or to equally distribute a power Pin between two powers Pout1 and Pout2.
The present invention more specifically relates to combiners/splitters having their distributed accesses (OUT1 and OUT2) in phase quadrature.
As compared with a coupler having the function of extracting a small part of a transmitted power for measurement purposes, a power combiner/splitter should respect parameters of phase balance and amplitude balance between the distributed paths.
FIG. 2 very schematically shows in the form of blocks a conventional example of a radio-frequency transmit circuit using a combiner (combiner-assembled block 1 of FIG. 1). Combiner 1 is interposed between the outputs OUT0 and OUT90, phase-shifted by 90° with respect to each other, of two power amplifiers 11 and 12 (PA) of a radio-frequency transmit head 10. Impedance matching circuits 13 and 14 (MATCH) shown in dotted lines may be interposed between amplifiers 11 and 12 and accesses OUT1 and OUT2 of the combiner. Each amplifier 11, 12 receives a radio-frequency signal RF0, RF90 originating from a phase-shift circuit 13 which itself receives two differential radio-frequency signals RFin+ and RFin− to be transmitted. Circuit 10 is supplied by a generally D.C. voltage Valim.
Combiner 1 adds signals OUT0 and OUT90 to form a signal IN sent onto an antenna 16 for transmission. A coupler may be added to the combiner to extract data proportional to the power POUT transmitted on access IN to possibly adjust the gains of amplifiers 11 and 12.
The same type of architecture may be used for a receive chain. In this case, the combined access (IN) is used as an input terminal while the two distributed accesses (OUT1 and OUT2) are used as phase-shifted output terminals (in phase quadrature) towards two receive inputs of a radio-frequency receive head.
To economize the power consumed by the amplification circuits (in transmit or receive mode), the signals are most often distributed in two paths in phase quadrature. Thereby, combiners/splitters are generally in phase quadrature for the distributed accesses.
The forming of combiners/splitters may use techniques said to be with local elements (association of inductive and capacitive elements) or with distributed or coupled lines (conductive lines arranged sufficiently close to each other to generate an electromagnetic coupling).
The present invention more specifically applies to combiners/splitters with distributed lines.
FIG. 3 shows a conventional example of a combiner/splitter formed in a distributed line technology. A first conductive line 21 connects combined access terminal IN to one, OUT1, of the distributed access terminals. A second conductive line, 22, connects a second distributed access terminal OUT2 to a terminal ISO left floating. Lines 21 and 22 are parallel and are generally formed by using planar track technologies of the type used in printed circuits. If, as shown, terminal OUT2 is on the side of terminal IN, the distributed accesses are in phase quadrature. If terminal OUT2 had been in the place of terminal ISO, the distributed accesses would be in phase.
To obtain the combiner/splitter effect, the coupler thus formed must be at 3 dB so that the power of terminal IN is distributed by half on each of terminals OUT1 and OUT2. In the architecture of FIG. 3, the length of each of lines 21 and 22 should correspond to one quarter of the wavelength (λ/4) of the work frequency of the combiner/splitter, that is, to one quarter of the wavelength of the central frequency of its bandwidth.
A disadvantage of a conventional combiner/splitter such as illustrated in FIG. 3 is its bulk for high frequencies which makes it in practice impossible to use in integrated circuits. For example, for a frequency on the order of one gigahertz now corresponding to the frequency bands used in mobile telephony, lines 21 and 22 should exhibit lengths of 34 mm each.
Another disadvantage is that this length of the conductive lines generates high network losses.
It should be noted that a combiner/splitter is fundamentally different from a balun (balanced/unbalanced) transformer which comprises a common-mode access and two differential-mode accesses. In particular, a balun does not enable obtaining a quadrature phase-shift, which is necessary in combiners to which the present invention applies. Further, distributed-line baluns are bulky since they have wavelengths equal to one quarter of the wavelength.